US 7165959 B2
An apparatus for casting a patterned surface on both sides of a web. The apparatus includes a first patterned roll; a second pattered roll; and a means for rotating the first and second patterned rolls such that their patterns are transferred to opposite sides of the web while it is in continuous motion. During this process, their patterns are maintained in continuous registration to within at least 100 microns, and preferably at least 20 microns.
1. An apparatus for casting a patterned surface on both sides of a web, the apparatus comprising:
a first patterned roll;
a second patterned roil;
means for rotating the first and second patterned rolls such that their patterns are transferred to opposite sides of the web while it is in continuous motion, and said patterns are maintained in continuous registration on said opposite sides of the web to within 50 microns.
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The invention relates generally to the continuous casting of material onto a web, and more specifically to the casting of articles having a high degree of registration between the patterns cast on opposite sides of the web.
In the fabrication of many articles, from the printing of newspapers to the fabrication of sophisticated electronic and optical devices, it is necessary to apply some material that is at least temporarily in liquid form to opposite sides of a substrate. It is often the case that the material applied to the substrate is applied in a predetermined pattern; in the case of e.g. printing, ink is applied in the pattern of letters and pictures. It is common in such cases for there to be at least a minimum requirement for registration between the patterns on opposite sides of the substrate.
When the substrate is a discrete article such as a circuit board, the applicators of a pattern may usually rely on an edge to assist in achieving registration. But when the substrate is a web and it is not possible to rely on an edge of the substrate to periodically refer to in maintaining registration, the problem becomes a bit more difficult. Still, even in the case of webs, when the requirement for registration is not severe, e.g. a drift out of perfect registration of greater than 100 microns is tolerable, mechanical expedients are known for controlling the material application to that extent. The printing art is replete with devices capable of meeting such a standard.
However, in some products having patterns on opposite sides of a substrate, a much more accurate registration between the patterns is required. In such a case, if the web is not in continuous motion, apparatuses are known that can apply material to such a standard. And if the web is in continuous motion, if it is tolerable, as in e.g. some types of flexible circuitry, to reset the patterning rolls to within 100 microns, or even 5 microns, of perfect registration once per revolution of the patterning rolls, the art still gives guidelines about how to proceed.
However, in e.g. optical articles such as brightness enhancement films, it is required for the patterns in the optically transparent polymer applied to opposite sides of a substrate to be out of registration by no more than a very small tolerance at any point in the tool rotation. Thus far, the art is silent about how to cast a patterned surface on opposite sides of a web that is in continuous motion so that the patterns are kept continuously, rather than intermittently, in registration within 100 microns.
One aspect of the present disclosure is directed to an apparatus for casting a patterned surface on both sides of a web while keeping a much finer registration between the patterns that has been possible in the past. The apparatus includes a first patterned roll and a second patterned roll for applying the patterns to the web. Also included is a means for rotating the first and second patterned rolls such that their patterns are transferred to opposite sides of the web while it is in continuous motion, and their patterns are maintained in continuous registration to within 100 microns. In another embodiment, a registration accuracy of within 50 microns can be accomplished, and in yet another embodiment, a registration accuracy of within 10 microns is possible.
In the context of this disclosure, “registration,” means the positioning of structures in a set location in relation to the edge of a web and to other structures on the opposite side of the same web.
In the context of this disclosure, “web” means a sheet of material having a fixed dimension in one direction and either a predetermined or indeterminate length in the orthogonal direction.
In the context of this disclosure, “continuous registration,” means that at all times during rotation of first and second patterned rolls the degree of registration between structures on the rolls is better than a specified limit.
In the context of this disclosure, “microreplicated” or “microreplication” means the production of a microstructured surface through a process where the structured surface features retain an individual feature fidelity during manufacture, from product-to-product, that varies no more than about 100 micrometers.
In the several figures of the attached drawing, like parts bear like reference numerals, and:
Generally, the invention of the present disclosure is a system and method for producing two-sided microreplicated structures with side-to-side registration of better than about 100 microns, and preferably better than 50 microns, and more preferably less than 25 microns, and most preferably less than 5 microns. The system generally includes a first patterning assembly and a second patterning assembly. Each respective assembly creates a microreplicated pattern on a respective surface of a web having a first and a second surface. A first pattern is created on the first side of the web and a second pattern is created on the second surface of the web.
Each patterning assembly includes means for applying a coating, a patterning member, and a curing member. Typically, patterning assemblies include patterned rolls and a support structure for holding and driving each roll. Coating means of the first patterning assembly dispenses a first curable coating material on a first surface of the web. Coating means of the second patterning assembly dispenses a second curable coating material on a second surface of the web, wherein the second surface is opposite the first surface. Typically, first and second coating materials are of the same composition.
After the first coating material is placed on the web, the web passes over a first patterned member, wherein a pattern is created in the first coating material. The first coating material is then cured or cooled to form the first pattern. Subsequently, after the second coating material is placed on the web, the web passes over a second patterned member, wherein a pattern is created in the second coating material. The second coating material is then cured to form the second pattern. Typically, each patterned member is a microreplicated tool and each tool typically has a dedicated curing member for curing the material. However, it is possible to have a single curing member that cures both first and second patterned materials. Also, it is possible to place the coatings on the patterned tools.
The system also includes means for rotating the first and second patterned rolls such that their patterns are transferred to opposite sides of the web while it is in continuous motion, and said patterns are maintained in continuous registration on said opposite sides of the web to better than about 100 microns.
An advantage of the present invention is that a web having a microreplicated structure on each opposing surface of the web can be manufactured by having the microreplicated structure on each side of the web continuously formed while keeping the microreplicated structures on the opposing sides registered generally to within 100 microns of each other, and typically within 50 microns, and more typically within 20 microns, and most typically within 5 microns.
Referring now to
Accurate tension control of the web 122 is required to achieve the best results the invention is capable of, so the web 122 is directed over a tension-sensing device (not shown). In situations where it is desirable to use a liner web to protect the web 122, the liner web is typically separated at the unwind spool and directed onto a liner web wind-up spool (not shown). The web 122 is typically directed via an idler roll to a dancer roller for precision tension control. Idler rollers direct the web 122 to a position between nip roller 154 and first coating head 156.
In the depicted embodiment, first coating head 156 is a die coating head. However, other coating methods can be adapted to the apparatus, as one of ordinary skill in the art will appreciate. The web 122 then passes between the nip roll 154 and first patterned roll 160. The first patterned roll 160 has a patterned surface 162, and when the web 122 passes between the nip roller 154 and the first patterned roll 160 the material dispensed onto the web 122 by the first coating head 156 is shaped into a negative of patterned surface 162.
While the web 122 is in contact with the first patterned roll 160, material is dispensed from second coating head 164 onto the other surface of web 122. In parallel with the discussion above with respect to the first coating head 156, the second coating head 164 is also a die coating arrangement including a second extruder (not shown) and a second coating die (not shown). In some embodiments, the material dispensed by the first coating head 156 is a composition including a polymer precursor and intended to be cured to solid polymer with the application of ultraviolet radiation.
Material that has been dispensed onto web 122 by the second coating head 164 is then brought into contact with second patterned roll 174 with a second patterned surface 176. In parallel with the discussion above, in some embodiments, the material dispensed by the second coating head 164 is a composition including a polymer precursor and intended to be cured to solid polymer with the application of ultraviolet radiation.
At this point, the web 122 has had a pattern applied to both sides. A peel roll 182 may be present to assist in removal of the web 122 from second patterned roll 174. Typically, web tension into and out of the casting apparatus is nearly constant.
The web 122 having a two-sided microreplicated pattern is then directed to a wind-up spool (not shown) via various idler rolls. If an interleave film is desired to protect web 122, it is typically provided from a secondary unwind spool (not shown) and the web and interleave film are wound together on the wind-up spool at an appropriate tension.
In the example embodiment shown, the tool roller 662 diameter is typically smaller than its motor 633 diameter. To accommodate this arrangement, the two tool roller assemblies 610, 710 are installed as mirror images in order to be able to bring the two tool rollers 662, 762 together as shown in
Reduction or elimination of shaft resonance is important as this is a source of registration error allowing pattern position control within the specified limits. Using a coupling 640 between the motor 633 and shaft 650 that is larger than general sizing schedules specify will also reduce shaft resonance caused by more flexible couplings. Bearing assemblies 660 are located in various locations to provide rotational support for the motor arrangement.
A second curable liquid layer 481 is coated on the opposite side of the web 422 using a second side extrusion die 464. The second layer 481 is pressed into the second patterned tool roller 474 and the curing process repeated for the second coating layer 481. Registration of the two coating patterns is achieved by maintaining the tool rollers 460, 474 in a precise angular relationship with one another, as will be described hereinafter.
In the example embodiment shown, first and second patterns 562, 576 are the same pattern, though the patterns may be different. As a web 522 passes over the first roll 560, a first curable liquid (not shown) on a first surface 524 is cured by a curing light source 525 near a first region 526 on the first patterned roll 560. A first microreplicated patterned structure 590 is formed on the first side 524 of the web 522 after the liquid is cured. The first patterned structure 590 is a negative of the pattern 562 on the first patterned roll 560. After the first patterned structure 590 is formed, a second curable liquid 581 is dispensed onto a second surface 527 of the web 522. To insure that the second liquid 581 is not cured prematurely, the second liquid 581 is isolated from the first curing light 525, typically by a locating the first curing light 525 so that it does not fall on the second liquid 581. Alternatively, shielding means 592 can be placed between the first curing light 525 and the second liquid 581. Also, the curing sources can be located inside their respective patterned rolls where it is impractical or difficult to cure through the web.
After the first patterned structure 590 is formed, the web 522 continues along the first roll 560 until it enters the gap region 575 between the first and second patterned rolls 560, 574. The second liquid 581 then engages the second pattern 576 on the second patterned roll and is shaped into a second microreplicated structure, which is then cured by a second curing light 535. As the web 522 passes into the gap 575 between first and second patterned rolls 560, 574, the first patterned structured 590, which is by this time substantially cured and bonded to the web 522, restrains the web 522 from slipping while the web 522 begins moving into the gap 575 and around the second patterned roller 574. This removes web stretching and slippages as a source of registration error between the first and second patterned structures formed on the web.
By supporting the web 522 on the first patterned roll 560 while the second liquid 581 comes into contact with the second patterned roll 574, the degree of registration between the first and second microreplicated structures 590, 593 formed on opposite sides 524, 527 of the web 522 becomes a function of controlling the positional relationship between the surfaces of the first and second patterned rolls 560, 574. The S-wrap of the web around the first and second patterned rolls 560, 574 and between the gap 575 formed by the rolls minimizes effects of tension, web strain changes, temperature, microslip caused by mechanics of nipping a web, and lateral position control. Typically, the S-wrap maintains the web 522 in contact with each roll over a wrap angle of 180 degrees, though the wrap angle can be more or less depending on the particular requirements.
To increase the degree of registration between the patterns formed on opposite surfaces of a web, it preferred to have a low-frequency pitch variation around the mean diameter of each roll. Typically, the patterned rolls are of the same mean diameter, though this is not required. It is within the skill and knowledge of one having ordinary skill in the art to select the proper roll for any particular application.
Because the features sizes on the microreplicated structures on both surfaces of a web are desired to be within fine registration of one another, the patterned rolls need to be controlled with a high degree of precision. Cross-web registration within the limits described herein can be accomplished by applying the techniques used in controlling machine-direction registration, as described hereinafter. Control of registration in the machine direction is required, which heretofore has not been achieved in two-sided microreplicated webs. For example, to achieve about 10 microns end-to-end feature placement on a 10-inch circumference patterned roller, each roller must be maintained within a rotational accuracy of ±32 arc-seconds per revolution. Control of registration becomes more difficult as the speed the web travels through the system is increased.
Applicants have built and demonstrated a system having 10-inch circular patterned rolls that can create a web having patterned features on opposite surfaces of the web that are registered to within 2.5 microns. Upon reading this disclosure and applying the principles taught herein, one of ordinary skill in the art will appreciate how to accomplish the degree of registration for other microreplicated surfaces.
Motor arrangement 900 communicates with a control arrangement 965 to allow precision control of the patterned roll 960. Control arrangement 965 includes a drive module 966 and a program module 975. The program module 975 communicates with the drive module 966 via a line 977, for example, a SERCOS fiber network. The program module 975 is used to input parameters, such as set points, to the drive module 966. Drive module 966 receives input 480 volt, 3-phase power 915, rectifies it to DC, and distributes it via a power connection 973 to control the motor 910. Motor encoder 912 feeds a position signal to control module 966. The secondary encoder 950 on the patterned roll 960 also feeds a position signal back to the drive module 966 via to line 971. The drive module 966 uses the encoder signals to precisely position the patterned roll 960. The control design to achieve the degree of registration is described in detail below.
In the example embodiments shown, each patterned roll is controlled by a dedicated control arrangement. Dedicated control arrangements cooperate to control the registration between first and second patterned rolls. Each drive module communicates with and controls its respective motor assembly.
Various options are available for co-coordinating the two axes such as master/slave-type and parallel configurations, which was used in Applicants' system.
The control arrangement in the system built and demonstrated by Applicants include the following. To drive each of the patterned rolls, a high performance, low cogging torque motor with a high-resolution sine encoder feedback (512 sine cycles×4096 drive interpolation >>2 million parts per revolution) was used, model MHD090B-035-NGO-UN, available from Bosch-Rexroth (Indramat). Also the system included synchronous motors, model MHD090B-035-NGO-UN, available from Bosch-Rexroth (Indramat), but other types, such as induction motors could also be used. Each motor was directly coupled (without gearbox or mechanical reduction) through an extremely stiff bellows coupling, model BK5-300, available from R/W Corporation. Alternate coupling designs could be used, but bellows style generally combines stiffness while providing high rotational accuracy. Each coupling was sized so that a substantially larger coupling was selected than what the typical manufacturers specifications would recommend. Additionally, zero backlash collets or compressive style locking hubs between coupling and shafts are preferred. Each roller shaft was attached to an encoder through a hollow shaft load side encoder, model RON255C, available from Heidenhain Corp., Schaumburg, Ill. Encoder selection should have the highest accuracy and resolution possible, typically greater than 32 arc-sec accuracy. Applicants' design, 18000 sine cycles per revolution were employed, which in conjunction with the 4096 bit resolution drive interpolation resulted in excess of 50 million parts per revolution resolution giving a resolution substantially higher than accuracy. The load side encoder had an accuracy of +/−2 arc-sec; maximum deviation in the delivered units was less than +/−1 arc-sec.
Preferably, each shaft is designed to be as large a diameter as possible and as short as possible to maximize stiffness, resulting in the highest possible resonant frequency. Precision alignment of all rotational components is desired to ensure minimum registration error due to this source of registration error. One of ordinary skill in the art will recognize that there are various ways to reduce registration error due to alignment of the rotational components.
The control strategy for each axis is implemented as follows:
The top path 1151 is the feed forward section of control. The control strategy includes a position loop 1110, a velocity loop 1120, and a current loop 1130. The position reference 1111 is differentiated, once to generate the velocity feed forward terms 1152 and a second time to generate the acceleration feed forward term 1155. The feed forward path 1151 helps performance during line speed changes and dynamic correction.
The position command 1111 is subtracted from current position 1114, generating an error signal 1116. The error 1116 is applied to a proportional controller 1115, generating the velocity command reference 1117. The velocity feedback 1167 is subtracted from the command 1117 to generate the velocity error signal 1123, which is then applied to a PID controller. The velocity feedback 1167 is generated by differentiating the motor encoder position signal 1126. Due to differentiation and numerical resolution limits, a low pass Butterworth filter 1124 is applied to remove high frequency noise components from the error signal 1123. A narrow stop band (notch) filter 1129 is applied at the center of the motor-roller resonant frequency. This allows substantially higher gains to be applied to the velocity controller 1120. Increased resolution of the motor encoder also would improve performance. The exact location of the filters in the control diagram is not critical; either the forward or reverse path are acceptable, although tuning parameters are dependent on the location.
A PID controller could also be used in the position loop, but the additional phase lag of the integrator makes stabilization more difficult. The current loop is a traditional PI controller; gains are established by the motor parameters. The highest bandwidth current loop possible will allow optimum performance. Also, minimum torque ripple is desired.
Minimization of external disturbances is important to obtaining maximum registration. This includes motor construction and current loop commutation as previously discussed, but minimizing mechanical disturbances is also important. Examples include extremely smooth tension control in entering and exiting web span, uniform bearing and seal drag, minimizing tension upsets from web peel off from the roller, uniform rubber nip roller. In the current design a third axis geared to the tool rolls is provided as a pull roll to assist in removing the cured structure from the tool.
The web material can be any suitable material on which a microreplicated patterned structure can be created. Examples of web materials are polyethylene terephthalate, polymethyl methacrylate, or polycarbonate. The web can also be multi-layered. Since the liquid is typically cured by a curing source on the side opposite that on which the patterned structure is created, the web material must be at least partially translucent to the curing source used. Examples of curing energy sources are infrared radiation, ultraviolet radiation, visible light radiation, microwave, or e-beam. One of ordinary skill in the art will appreciate that other curing sources can be used, and selection of a particular web material/curing source combination will depend on the particular article (having microreplicated structures in registration) to be created.
An alternative to curing the liquid through the web would be to use a two part reactive cure, for example, an epoxy, which would be useful for webs that are difficult to cure through, such as metal web or webs having a metallic layer. Curing could be accomplished by in-line mixing of components or spraying catalyst on a portion of the patterned roll, which would cure the liquid to form the microreplicated structure when the coating and catalyst come into contact.
The liquid from which the microreplicated structures are created is typically a curable photopolymerizable material, such as acrylates curable by UV light. One of ordinary skill in the art will appreciate that other coating materials can be used, for example, polymerizable material, and selection of a material will depend on the particular characteristics desired for the microreplicated structures. Similarly, the particular curing method employed is within the skill and knowledge of one of ordinary skill in the art. Examples of curing methods are reactive curing, thermal curing, or radiation curing.
Examples of coating means that useful for delivering and controlling liquid to the web are, for example, die or knife coating, coupled with any suitable pump such as a syringe or peristaltic pump. One of ordinary skill in the art will appreciate that other coating means can be used, and selection of a particular means will depend on the particular characteristics of the liquid to be delivered to the web.
Various modifications and alterations of the present invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this invention is not limited to the illustrative embodiments set forth herein.